A conventional MEMS microphone typically has a static substrate/backplate that, together with a flexible diaphragm, forms a variable capacitor. For example, the backplate may be formed from single crystal silicon (e.g., a part of a silicon-on-insulator wafer or a bulk silicon wafer), while the diaphragm is formed from deposited polysilicon. To facilitate operation, the backplate has a plurality of through-holes that lead to a backside cavity. The backside cavity often has a generally rounded shape.
During fabrication, the interior walls of the through-holes sometimes have an oxide that must be removed. To remove this oxide, some processes often immerse the backplate in a liquid bath of hydrofluoric acid. Specifically, such processes known to the inventors typically place the apparatus that is to become the microphone into the liquid bath in a “substrate first” manner; namely, the diaphragm (which is not necessarily immersed) is vertically above the substrate within the bath receptacle, although other configurations may be used.
Surprisingly, despite the relatively large size of the backside cavity, the liquid often cannot penetrate into the backside cavity and thus, cannot access the through-holes. Specifically, the inventors have noticed that the liquid often forms a bubble around the opening of the backside cavity. Accordingly, absent some additional measures, such as directly applying a mechanical force to the bubble (e.g., blowing air toward the backside cavity at a sufficient velocity to break the bubble), the liquid does not enter the through-holes and, consequently, cannot adequately remove the oxide.
In addition, a generally rounded backside cavity often does not adequately support the backplate. When not adequately supported, a backplate undesirably may droop or bow.